Engineers are developing vehicles utilizing diverse sources of energy. Many of these innovative energy sources are not applied directly to a vehicle powertrain, but are instead first transformed into electric power and harnessed through a power inverter module coupled to an electric motor. Power inverter modules allow the transformation of direct current electrical power to alternating current electrical power for use by the motor. While alternating current is desirable for use in the electric motor for a wide variety of benefits, the generation of alternating current in the power inverter module also creates drawbacks, including the creation of stray radio frequency (“RF”) signal noise.
An electromagnetic field is generated whenever there is a change in voltage created in a conductive medium. As is well known in the art, power inverter modules utilize high-speed switching to generate the alternating current power source for the electric motor, and this switching may generate rises in voltage with durations measured in nanoseconds. The voltage changes generated by the power inverter module travel through the electric motor and create a circuit through attached conductive pieces. As in any electrical circuit, the electrical charge created at the generating source will seek to close the circuit through the path of least resistance. However, if the circuit is open, meaning no conductive path exists for the electrical charge to flow back to the generating source, the voltage changes generated by the source propagate through any attached conductive medium before creating an electrical field across the gap to complete the circuit. This propagation of voltage changes causes any attached conductive medium to act like a broadcasting antenna, wherein an electromagnetic field in the form of RF signal noise is created by the propagated voltage changes. Stray RF signal noises are stray radio signals or interference patterns that may impact or degrade the performance of other nearby electrical equipment. One way to reduce the generation of stray RF signal noise is to reduce RF signal generating voltage propagation by providing a conductive return path for charge created in high-speed switching to return to the generating source.
In vehicular powertrain applications, electric motors are found in conductive attachment to the drive train, frequently taking the form a transmission output shaft extending from a transmission housing. This transmission output shaft spins and, therefore, cannot come into solid contact with the transmission housing. A metallic bearing is frequently used to create an axially fixed but rotating union between the transmission housing and the transmission output shaft. While this contact provided by the bearing may be in some conditions be adequate as a conductive contact between the transmission housing and the transmission output shaft, a thin film of transmission fluid actually exists as a lubricant between the mating surfaces of the bearing. A small gap such as one created by a lubrication film will, in many applications, causes a small arc to form across the film, thereby closing the circuit and eliminating any potential issues of RF signal noise. However, testing has shown that recent developments in power inverter modules utilizing higher switching speeds have resulted in the gap caused by the film becoming more significant. The voltage rise in the power inverter module and any attached conductive medium is so rapid that the voltage propagates through the conductive medium before an arc may form to cross the gap.